A fail-safe "AND" logic operation-oscillation circuit which oscillates only when the level of an input signal (or a plurality of signals) has a predetermined value and which does not oscillate when an input signal does not have a predetermined high level or when a problem takes place in the input signal is proposed, for example, in Japanese Utility Model Application Laid-Open Specification No. 57-4764.
This "AND" logic operation-oscillation circuit is fabricated by using a plurality of semiconductor elements and a plurality of resistors so that oscillation is not caused when a problem such as breaking or short circuit takes place in circuit elements, and in general, the input signal level is set at a value higher than the power source voltage.
In an integrated circuit fabricated by forming a plurality of circuits with circuit elements on a semiconductor substrate, a resistor portion ordinarily has a structure as shown in FIG. 1. Namely, an N-type region 2 is formed on a P-type substrate 1 so that the region 2 is electrically insulated from the substrate 1, and a P-type diffused resistor 3 is formed on the N-type region 2. In order to produce an electric insulation between the N-type region 2 and the P-type diffused resistor 3, a maximum voltage in the integrated circuit is applied to the N-type region 2 through terminals a and b to cause reverse bias in the diode junction between the region 2 and the resistor 3. An oxide film 4 of silicon dioxide (SiO.sub.2) is formed on the surface of the entire assembly.
In the case where an "AND" logic operation-oscillation circuit is integrated in the above-mentioned manner, in which the level of an input signal is set at a value higher than the power source voltage as in the above-mentioned "AND" logic operation-oscillation circuit if a voltage of an input signal higher than the power source voltage is applied to the resistor portion connected to an input signal terminal, the diode junction between the N-type region 2 and P-type diffused resistor 3 of the resistor portion is biased in the forward direction of the electric current and the function of the resistor is not performed. This disadvantage will be eliminated if the voltage to be applied for reverse biasing of the above-mentioned diode junction is made equal to the maximum voltage of the input signal. In this case, however, if a trouble of short circuit problem is caused in the diode junction, the state is same as the state where an input signal of a level higher than the power source voltage is put in, and in spite of the absence of an input signal, an oscillating output is produced and no fail-safe effect is attained.